1. Field of the Invention
The present invention relates to data detectors, and more particularly to receivers for synchronizing and detecting the transmission of data between digital systems or subsystems.
2. Description of the Prior Art
In the transmission of digital data over a communication channel, a coded waveform is utilized that synchronizes and validates each data word. A synchronizing waveform, which detects the beginning of each data word, is followed by a series of data waveforms, each of which detects the identity of a data bit of such word. These coded waveforms are detected by a receiver that validates the synchronizing and data waveforms.
The coded waveforms utilized are typically a series of bipolar pulses, the polarity of which differs in accordance with the data bit identity. Such bipolar waveforms may be of different shapes, such as sawtooth, square, or sinewave, for example. The synchronizing waveform, of course, must be different than the data waveforms. A coded waveform that offers definite advantages in the transmission of data is the well known Manchester coded waveform that is a bi-phase-level data waveform that immediately follows a synchronizing waveform. For the data bit waveforms, the state of the signal during the first 180.degree. phase of each bit period corresponds to the logic state being transmitted. For example, if the data signal is "high" during the first 180.degree., a logical "one" is being transmitted. If the data signal is "low" during the first 180.degree., a logical "zero" is being transmitted. Thus, if "f" is the frequency of transmission, a pattern of all "one's" or a pattern of "zero's" represents a frequency of "f". A pattern of alternate "one's" and "zero's" represents a frequency of one-half "f". The synchronization waveform is a duration of three bits, resulting in a frequency of one-third "f". No allowable combination of bit waveforms can produce the synchronization waveform; thus, it is readily distinguishable. In a valid data portion of a Manchester code, there is always a zero crossover at each odd multiple of 180.degree.; but, there may or may not be a zero crossover at even multiples of 180.degree.. For example, if the first data bit is a "zero", then the first or odd 180.degree. goes from low to high, crossing through zero voltage. If the next data bit is "zero", the even or second 180.degree. goes from low to high, again crossing through zero voltage. But, if the next or second data bit is a "one" the even or second 180.degree. stays high, and does not go through zero until the beginning of the third data bit. Stated another way, there is always a transition of the waveform through zero voltage at the mid-position of each bit, but not necessarily at the end of each bit.
Difficulty arises when the coded waveforms must be received over a noisy transmission channel, which obscures the identity and location of the synchronizing and data bit codes rendering reception inaccurate.
Heretofore, noise accommodation was attempted by several methods and devices. One example is the use of a matched filter, which provides significant noise improvement. This filter is matched to a preferred waveform to be detected in the sense that the response to a unit impulse is equal to the said preferred waveform reversed in time. However, the output of such matched filter does not resemble the input waveform. For example, square waves are converted into sawtooth waveforms; and the signal-to-noise ratio is the greatest at the peak of the waveform. Thus, the data detection must be accomplished in the area of the peak. This requires the precise location of such peak; and increases the problem of synchronous time location.
Further, the use of a general analog filter has certain advantages in that it passes relevant portions of the coded waveform and overcomes the location problem attendant with the use of the matched filter. However, the bandwidth of the analog filter had to be such that the signal-to-noise ratio improvement factor was only in the neighborhood of 3 to 5 decibels, for example. This signal-to-noise ratio is substantially inferior to the matched filter technique.
Another prior art method and apparatus for noise improvement, particularly in radar systems, is to utilize what is termed "coincidence detection" or sometimes referred to as "majority logic". In this method, the clocking waveform of a much higher frequency than the coded waveform is utilized to detect the plurality of samples of each positive and negative portion of the waveform; that is, during each code pulse, a plurality of samples N of the waveform is detected. If a predetermined number k is above a certain threshold in the case of the positive waveforms, or below a certain threshold in the case of the negative waveform, the synchronous and data pulses are deemed to be valid. This coincidence detection improves the signal-to-noise ratio effectively which improves the receiver's ability to detect that a valid signal is present. However, the problem of where the signal begins and ends is still present. This is because the k out of the N samples could have occurred during any portion of the coded waveform bit; that is, the beginning, the center, or the end. Still another technique utilized in such receivers is the detection of consecutive samples above a threshold for the positive waveform and the detection of consecutive samples below a threshold for the negative waveform. With this arrangement, it is possible to determine accurately the location of the signal. However, such signal is detected as invalid if a single noise spike should occur and destroy such consecutive detection. In order to reduce the probability of such occurrence, selected samples of the waveform are detected. For example, the second sample and the fifth sample, for data bits that are clocked six times, are selected. This detects the mid-position of the first half and the mid-position of the second half of the data bit. For a "one", the second sample must be detected above a predetermined positive threshold and the fifth sample must be detected below a predetermined negative threshold. For a "zero" the second sample must be detected below the negative threshold and the fifth sample must be detected above the positive threshold to validate the bit. If a noise spike occurs during such second or fifth sample, the bit is destroyed. Although this consecutive and selective sampling precisely located the signal, such technique did not provide an adequate noise margin.
Therefore, it is desirable to provide an improved system and method of detecting synchronous and data pulses in a coded waveform, which provides a substantial improvement in accommodating transmission line noise over the systems and methods presently known, while at the same time providing for a precise location of the signals.